Increasing the density of integrated circuit devices generally requires reduction of the size of the transistors used in the devices in order to incorporate more transistors into the integrated circuit. To reduce the size of the transistors, it is generally desirable to reduce the area of the active regions in the integrated circuit. In addition, it generally is desirable to reduce the area of the device isolation regions used to separate the active regions of the integrated circuit.
One technique for forming device isolation regions involves the local oxidation of silicon (LOCOS). According to this technique, an insulating oxide layer is grown between active regions by thermally oxidizing silicon substrate regions between the active regions. However, as the oxidation process tends to extend laterally across the face of the substrate as well as vertically into the regions, so called "bird's beaks" may be produced at the edges of the active regions which may encroach on and undesirably narrow the active regions. In addition, the field oxide layers formed in wider device isolation regions tend to be thicker than those formed in narrower device isolation regions, making it difficult to achieve the desired oxide thickness in some areas of the substrate.
An alternative to the LOCOS techniques is a trench isolation method whereby a photolithographic mask is used to define boundaries of a trench on a substrate surface surrounding an active region and trenches are etched into the substrate according to the mask. The trenches are then filled with an insulating material such as a deposited silicon dioxide to form a device isolation region. To reduce separation between active regions, the trenches generally must be very narrow. However, the narrowness of trenches tends to be limited by the resolution available using conventional photolithography techniques, i.e., it is generally difficult to reliably produce a sufficiently narrow photoresist pattern. In addition, it may be difficult to form trenches sufficiently deep to provide the necessary isolation because of the difficulty of filling narrow, deep trenches without producing voids or other defects.
For this reason, trench isolation may be augmented by forming a local field oxide in conjunction with a filled trench. FIG. 1 illustrates active regions 12 defined on a semiconductor substrate using combined trench/LOCOS isolation according to the prior art, by an isolation region including a trench region 10 and a field region 13. As illustrated by FIG. 2, representing a cross-section of the substrate taken along a line D-D' of FIG. 1, a trench region 10 is formed between active regions 12 filled with a material 14 such as polysilicon or silicon dioxide, over which the field oxide region 13 is formed. The minimum width A of the isolation field region 14 defining active region 12 generally is the sum of twice the distance C between the edges of the trench 10 and the active regions 12, and the trench width B. Although reducing the width B of the trench region 10 can reduce the overall width A of the isolation region 14, the reduction achievable is generally limited by the minimum resolution of the photolithography. In addition, by using local oxidation, bird's beaks 16 may be formed which may encroach on the active regions 12.